awww, I don't have a problem with Rotastak...
I would have a young Syrian in them and have had my dwarfs in them...but once they are too big to get through the tubes, they get a different cage. I think as long as you have enough compartments then small furbabies are ok in them...and you can vary them too!!
Saying that, none of mine are in Rotastaks anymore...just because they are a pain in the botty to clean!!
Your little gerbilarium looks really cool...plenty of fun and variation for the gerbils!! they are cute!
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